dissertation
Uroš Luin (Author), Matjaž Valant (Mentor), Nejc Hodnik (Thesis defence commission member)

Abstract

Future high-capacity energy storage technologies are crucial for a highly renewable energy mix, and their mass deployment must rely on cheap and abundant materials, such as iron chloride. The iron chloride electrochemical cycle (ICEC), suitable for long-term grid energy storage using a redox potential change of Fe2+/Fe, involves the electrolysis of a highly concentrated aqueous FeCl2 solution yielding solid iron deposits. For the high overall energy efficiency of the cycle, it is crucial maximizing the energy efficiency of the electrolysis process. The thesis presents a study of the influence of electrolysis parameters on energy efficiency, performed in an industrial-type electrolyzer system. We studied the conductivity of the FeCl2 solution as a function of concentration and temperature and correlated it with the electrolysis energy efficiency as a function of current density. The contribution of the resistance polarization increases with the current density, causing a decrease in overall energy efficiency. The highest energy efficiency of 89 ±3 % was achieved using 2.5 mol dm-3 FeCl2 solution at 70 °C and a current density of 0.1 kA m-2. In terms of the energy input per Fe mass, this means 1.88 Wh g-1. The limiting energy input per mass of the Fe-deposit, calculated by extrapolating experimental results toward Eocell potential, was found to be 1.76 Wh g-1. For optimal long-duration electrolysis efficiency and performance, the optimal catholyte concentration range is 1-2 mol dm-3 FeCl2. We performed in situ X-ray absorption spectroscopy experimental studies to validate theoretical conclusions from literature related to the population and structure of Fe-species in the FeCl2 (aq) solution at different concentrations (1 - 4 mol dm-3) and temperatures (25 - 80 °C). This revealed that at low temperature and low FeCl2 concentration, the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five H2O at a distance of 2.095 (±0.005) Å. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical H2O is substituted by a Cl ion to yield a neutral Fe[Cl2(H2O)4]0. The transition from the charged Fe[Cl(H2O)5]+ to the neutral Fe[Cl2(H2O)4]0 causes a significant drop in the solution conductivity, which well correlates with the existing state-of-the-art conductivity models. An additional steric impediment of the electrolytic cell is caused by the predominant neutral species present in the catholyte solution at high concentration. This correlates with poor electrolysis performance at a very high catholyte concentration (4 mol dm-3 FeCl2), especially at high current densities (> 1 kA m-2). The neutral Fe[Cl2(H2O)4]0 complex negatively affects the anion exchange membrane ion (Cl-) transfer and lowers the concentration of electroactive species (Fe[Cl(H2O)5]+) at the cathode surface. The kinetics of hydrogen evolution from the reaction between Fe powder and HCl acid was studied under the first-order reaction condition. The activation energy was determined to be 55.3 kJ mol-1.

Keywords

ICEC;Power-to-Solid;energy storage;hydrogen;ferrous chloride;electrolysis;Fe deposition;efficiency;XAS;ionic species;ion association;dissertations;

Data

Language: English
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UNG FPŠ - Graduate School
Publisher: [U. Luin]
UDC: 621.311.1:620.1/.2(043.3)
COBISS: 149530115 Link will open in a new window
Views: 2386
Downloads: 37
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Other data

Secondary language: Slovenian
Secondary title: Učinkovitost tehnologije za shranjevanje omrežne električne energije na osnovi železokloridnega snovnega toka
Secondary abstract: Prihodnje tehnologije shranjevanja energije z visoko kapaciteto bodo ključne za visokega deleža obnovljivih virov v energetski mešanici. Za množično uporabo mora tehnologija temeljiti na poceni in obilnih materialih, kot je to železov klorid. Elektrokemijski cikel na osnovi železo-kloridnega snovnega toka je primeren za dolgoročno shranjevanje energije preko spremembe redoks potenciala (Fe2+/Fe). Vključuje elektrolizo visoko koncentrirane vodne raztopine FeCl2 z nalaganjem elementarnega železa na katodo. Za visoko celokupno energijsko učinkovitost cikla je ključnega pomena maksimiranje energetske učinkovitosti procesa elektrolize. V tej disertaciji je predstavljena študija vpliva parametrov elektrolize na energijsko učinkovitost, izvedena v pilotnem industrijskem elektrolizerskem sistemu. Proučevali smo prevodnost katolitne vodne raztopine FeCl2 v odvisnosti od koncentracije in temperature ter jo korelirali z energijsko učinkovitostjo elektrolize v odvisnosti od gostote toka. Zaradi upornosti elektrolizne celice se izgube z gostoto toka večajo, kar povzroči zmanjšanje energetske učinkovitosti. Najvišjo energetsko učinkovitost 89 ±3 % smo dosegli z uporabo 2,5 mol dm-3 FeCl2 (aq) raztopine pri 70 °C in gostoti toka 0,1 kA m-2. Dovedena energija na maso Fe znaša 1,88 Wh g-1. Minimalni ali limitirajoči vnos energije na maso Fe smo izračunali z ekstrapolacijo eksperimentalnih rezultatov proti potencialu celice (Eocell) in znaša 1,76 Wh g-1. Za stabilno dolgotrajno učinkovitost in zmogljivost elektrolize je ugotovljeno optimalno območje koncentracije katolita 1 - 2 mol dm-3 FeCl2. Izvedli smo (in situ) eksperimentalne študije z uporabo sinhrotronske rentgenske absorpcijske spektroskopije za ovrednotenje literaturnih teoretičnih zaključkov v zvezi s populacijo in strukturo Fe zvrsti v vodni raztopini FeCl2 pri različnih koncentracijah (1 - 4 mol dm-3) in temperaturah (25 - 80 °C). Pri nizki temperaturi in nizki koncentraciji FeCl2 ima kompleks oktaedrično prvo koordinacijsko sfero okoli Fe, ki jo zaseda en Cl ion na razdalji 2,33 (±0,02) Å in pet H2O na razdalji 2,095 (±0,005) Å. Struktura ionskega kompleksa se postopoma spreminja z višanjem temperature in/ali koncentracije. Vršna H2O je nadomeščen z ionom Cl, kar rezultira nevtralni Fe[Cl2(H2O)4]0. Prehod iz nabitega Fe[Cl(H2O)5]+ v nevtralni Fe[Cl2(H2O)4]0 kompleks povzroči znaten padec prevodnosti raztopine, kar dobro korelira z obstoječimi najsodobnejšimi modeli prevodnosti. Pri visokih koncentracijah katolita povzročajo v elektrolitski celici dodatno sterično oviro prevladujoče nevtralne vrste. To sovpada s slabo elektrolizno učinkovitostjo pri zelo visoki koncentraciji katolita (4 mol dm-3 FeCl2), zlasti pri visokih gostotah toka (> 1 kA m-2). Nevtralni kompleks Fe[Cl2(H2O)4]0 negativno vpliva na prenos ionov (Cl-) preko anionsko izmenjevalne membrane in znižuje koncentracijo elektro-aktivnih zvrsti (Fe[Cl(H2O)5]+) na površini katode. Kinetiko razvijanja vodika iz reakcije med Fe prahom in HCl kislino smo proučevali pod pogoji reakcije prvega reda. Ugotovljena aktivacijska energija znaša 55,3 kJ mol-1.
Secondary keywords: shranjevanje energije v snovi;vodik;železov klorid;elektroliza;nalaganje Fe;učinkovitost;XAS;ionske zvrsti;povezovanje ionov;prevodnost;disertacije;Električna energija;Disertacije;Shranjevanje;
URN: URN:SI:UNG
Type (COBISS): Doctoral dissertation
Thesis comment: Univ. v Novi Gorici, Fak. za podiplomski študij
Pages: XV, 132 str.
ID: 18330059